If Snape told the truth to Bella, that Dumbledore feared a relapse in the past teaching years of Snape, then Dumbledore thinks that Snape's interest in the Dart Arts is somewhat equivalent to any dependency we know of. For example, we all know that the best way for an ex-alcoholic to keep going on is to never touch a glass or bottle of alcohol ever again. We know that an ex-smoker should not smoke a cigarette ever again.  We know humans are frail when it comes to such dependencies and that the best remedy if not the only one is to not touch it anymore.  But is Dumbledore so worried about Snape that he won't let him teach what he loves best?  After all, who would be better to teach how to defend yourself from the Dark Arts than someone who has been through it before?  We know it takes an ex-alcoholic to help another because people simply can't understand the drive and the attraction of alcohol as well as an ex-drinker.  Therefore, I have always had my doubts about the real reasons or the whole truth of Snape never getting the DADA class. But I always thought it had to be BIG and have many consequences on the plot or Rowling wouldn't have mentioned it from the first book and on.  

Giving him the job now for the reason Snape just told Bellatrix (and apparently Voldemort)  doesn't make any sense! Why? Because now that the Dark Lord has risen again, it is even more dangerous for Snape to succumb to the Dark Arts!! It's as simple as that!  When you have an ex-alcoholic under your roof and that you kept him 15 years without being able to teach his favourite subject, Defense Against Alcohols and Drugs, because you thought he would relapse  that makes sense. What doesn't is that in your employee's 16th year of service,  you suddenly ask him to teach DAAD BUT  you also know your teacher's 'old pals' are in town and have reopened a liquor store downtown where beer, wine and spirit alcohols flow freely from their store public fountain!!  That would be insane! Therefore, my point of view is that the reason why Snape never got the job was not because Dumbledore feared a relapse. If Dumbledore really feared a relapse, then he wouldn't have given the job to Snape that year because Snape was the Order's best spy!  If you fear your spy will turn on you, you'll make sure he is monitored very closely.  Did you see anyone monitoring Snape closely? I didn't.  And you don't tempt your spy either by putting him in the kingdom of the Dark Arts!

Another point: Might we suggest that Snape also committed murder before and Dumbledore still trusts him? Not everyone would agree with this view – some would say that because a civil war is taking place among the wizarding community any kill is legitimate and does not count as murder. However I think this possible legal loophole is not needed because  I wonder a lot about this because it implies that in many people's opinion, Dumbledore would harbor a murderer. Not everyone would agree that under such circumstances, a wizard civil war sort of, that every 'kill'  was murder.  However, I doubt that any murder case could be proven in any way against Snape because the whole scheme of  'Snape warned Dumbledore because he realized he'd made an awful mistake that might lead to the death of the Potters' is presented as though it was the first time Snape exposed people to danger. People he knew might be more exact and I'm afraid we shall not know until Book 7 whether Snape killed other people he didn't know of and only felt guilty when the death sentence fell upon the Potters. From the point of view of warfare laws and Snape spilling the beans before the fall of the Dark Lord, yes,  Dumbledore could 'forgive' murder in Snape if he considered Snape was an ally he needed. Maybe those were desperate times and the headmaster had no choice but to trust Snape. However, if that had been so, he would not have kept Snape after the war and included him so diligently into the Order of the Phoenix years later, nor would he have kept someone with doubtful loyalties to teach his precious students.  How could Snape convince Dumbledore that he was in earnest?   Why did some Order members not know all there was to know about it while Snape knew so much? Why do Muggles liberate some prisoners earlier sometimes?

The answer my friends is simple: One would have to prove beyond any doubts that one felt guilty for his past actions and wanted to do anything they could to set it right!  In the Muggle world, it's hard knowing if someone is telling the truth. We have some truth detector but even those may lie at times.  But most of the time, when someone is genuine in his behaviour, it's identifiable. We do it plenty in Canada and though our Justice system makes mistakes at times, we're not a nation plagued by mad ex-prisoners who run free at night to kill innocent citizens. No, it happens but it's pretty rare.  In the Wizarding world however, they have ways to make sure someone truly is repentant and will take an oath to do all they can to make it up for you.  Veritaserum, Unbreakable Vow, branding people with your own Dark Mark, Legilimens, etc. There must be other ways that Rowling imagined but never mentioned, too. But if we take into account those we have and we consider that Dumbledore needed to make sure he could trust Snape, someone as incredibly high-profiled as the Headmaster of Hogwarts, creator of the Order of the Phoenix and most powerful wizard alive needed solid proof. Not only solid, indestructible proof that Snape was in earnest. Too many people were at risk if Dumbledore trusted Snape with vital information, it could mean the end of the Light and the victory of Dark Wizards, countless lives in the balance!  

Veritaserum: yes, Dumbledore used it without any qualms on Barty Crouch Jr so he would use it on Snape if he needed to.  Dumbledore would have asked Snape what he knew, what he didn't know, what he was ready to do to make it up to the Light and bring down Voldemort, etc. But Veritaserum is good only for the present, you would have to retest someone each year to make sure they weren't hiding something.  
Unbreakable Vow: yes, I love this theory because it's perfect!  No loopholes with this one: you do what you promised to or you die. That's it that's all!  Dumbledore would make Snape swear he would do anything to bring down the Dark Lord, that he would protect the Order of the Phoenix, that he didn't want to be a Death Eater thereof and that would prove that he was genuinely guilty.
A mark: No, somehow I can't imagine Dumbledore branding his mark on Snape ; )  What would it look like? A lemon drop?! Ha ha!
Legilimens: NO! I think Dumbledore used it but if he knew that Snape was an incredible Occlumens in Book 5, then if he had trusted Snape in the first place because of that, Dumbledore would retest Snape's loyalty through one of the first two means.  Dumbledore knows Snape's abilities are strong enough to withhold Voldemort's skills at Legilimens, so why would he use Legilimens on Snape?!  The Dark Lord is too full of himself, he's sure he is the best Legilimens in the world but he is mistaken since he can't detect Snape's lies, apparently.  Why would Dumbledore use this magic on Snape if he knows Snape can do that!! It doesn't make any sense, so it's a big NO for me.
Other methods: maybe, we don't know and I don't think we were given clues to any other type of magic as powerful as Veritaserum and the Unbreakable Vow.

Which narrows our options to 2:  Snape gave a rock-hard irrefutable proof of his guilt and repentance (nothing short of an Unbreakable Vow or Veritaserum) OR if Dumbledore used Legilimens on Snape even though he knows Snape is able to fool the Dark Lord with, it means that Dumbledore has gone mental for more than 15 years. Yes, if he trusted Snape without any magical and tangible proof of his honesty like good ole' veritaserum or the Unbreakable Vow while he handed over vital information about millions of helpless people to Snape, then we should consider Dumbledore to be as daffy as Looney the loon from Simpleton Village in the vicinity of Insanity-town who  just won for the 150th time in a row the Nutty Balmy Crackbrainy Dotty Batty Tournament held at the Daft, Demented & Wacky Festival down in Delusional country.  [I thank the series Black Adder for this inspiring name ; ) ]

Yes, ladies and gentlemen, either Dumbledore is a total wacko or Snape proved beyond the shadow of a doubt that he would work for Dumbledore (and the Light) from now on and until Snape dropped dead!  Which brings us this famous quote:  (HBP, chapter 25) "Dumbledore did not speak for a moment; he looked as though he was trying to make up his mind about something. At last he said, 'I am sure. I trust Severus Snape completely.'

Completely : having no exceptions or restrictions <he's a complete lunatic> -- see ABSOLUTE 2

Absolutely, totally, completely: all synonyms meaning that Dumbledore trusts Severus Snape for EVERYTHING HE TOLD AND ASKED SNAPE!  That is quite a lot to speak of someone like that.  He is sure that he believes in him completely. Wow!  That really is a powerful statement.  Dumbledore could have said anything else but he said this 'no matter what I trust Snape' kind of sentence. And that to me translates into "I even trust him with  all of our lives and the survival of the Order, hence of the Light". Isn't that either the greatest kind of faith or the greatest kind of misplaced trust on Earth!  Only Rowling shall tell which one it is!

It is also written that 'Dumbledore did not speak for a moment; he looked as though he was trying to make up his mind about something'.  What is this 'something'?  

"Do I explain Harry why I trust him so much because he made an Unbreakable Vow to me or do I simply state that I trust him completely? Will he be able to hold his tongue, temper and dreams if I tell him too much?"
"Do I tell Harry that I gave him Veritaserum when he came to me and turned a spy?"
"Do I reveal my plans to Harry, about knowing that Snape agreed to kill me on my orders?"
"Do I tell Harry that I only used Legilimens on Snape all of this time even though I know it's not the best way to test his loyalty? If I say that, Harry will lose faith in me and the Order and it may lead to worse problems."
"Do I tell Harry that I believed Snape's words when he came to me without any magic and that I'm just gambling on the fact that Snape is apparently working for us?

I say the three first are the ones and the red sentence applies to all of them as well, I simply didn't repeat it three times.  Dumbledore was thinking of two possible things: revealing what he knew to Harry (or why he shouldn't reveal anything) and thinking over his trust in Snape (or doubts if he still had any towards Snape). 

Revealing information to Harry (or trusting Harry with the truth):  Dumbledore needs Harry to trust him but Harry has something against Snape that he cannot yet overcome emotionally. This may explain why Dumbledore would hesitate to share such secret information, one piece of info that could mean Snape's death as well, because Harry would surely not be able to keep it for himself. Harry usually only hides from Ron and Hermione what he's ashamed of, but something like that was bound to be retold by Harry. It was because he is biased about Snape as a spy, as much as Snape is biased about Harry as a student.  But Snape always saved Harry in the past even though he dislikes him as a student, he does the right thing. Is Harry mature enough to do the right thing even though it appears to be a thorn in your side at first? I don't think so and if Dumbledore hesitated because he wasn't sure if he should tell Harry or not, I say he did the right thing.

The right thing though this withholding of information could cause Snape to be killed by the Order before the plot unfolds completely. Don't shout, don't pout, it may happen and you know it!  Just as Snape turning out bad can happen as well.  I wish I were as confident as I am about trusting Snape when it comes to his premature death. I do have the feeling that he's going to meet his end and be killed by the Order while undercover for them. Tragic but a very effective moral that would be. I know Rowling said she didn't care much about morals, that she would not insist on some lessons to be learnt from her books, but I know that if Snape dies while working for the Light and is killed by the Light, it will affect fewer readers.  If Snape turns out bad it will affect less readers however. That reminds me that I wanted to talk about these moral messages that Rowling said she didn't want to send out. Here it is (click here and another window will pop up, it's not too long)

Now the second point, trusting Snape or not?  

That's THE question most of you will say and I agree.  I may repeat myself at times but I want to be sure everyone follows my train of thought:
Dumbledore, unless he was senile after all and nobody but Lucius had noticed, trusts Snape for a very valid reason.  By that I mean that, again unless Dumbledore's a total loony, he will protect those under his care no matter what  because the Order is the only effective defense against the Death Eaters and Voldemort.  It's Good against Evil in an epic magical battle!  Therefore, Dumbledore needs to be certain of Snape's loyalties because he is the key figure in the middle. He is the main spy, Dumbledore has to know that Snape is an excellent Occlumens or else he would NEVER send Snape back to Voldemort while also working for the Order. I mean, Snape has been to Order meetings, to Grimauld Place and so on. He's Dumbledore's left arm almost and yet, even with all this precious and vital information, Dumbledore sends Snape out there to a maniac that excels at Legilimens. His certainty of Snape follows logically, given that Dumbledore is prepared to   send a very informed Snape into the enemy lines. If Dumbledore feared for even a second that Snape would betray him and the whole wizarding world which the headmaster had spent his whole life protecting, then he would not have allowed Snape to leave the Infirmary that fateful night at the very end of Book 4. Dumbledore would have petrified Snape on the spot instead of sending him to the Dark Lord where he would spill all their secrets.  Dumbledore knows that sacrifices are necessary at times and if Snape was sure to be a turncoat again, he wouldn't have made it out of Hogwarts that night.  (more about this farther below)

Snape quite possibly knows all the secrets because he's been there all this time, he knows the staff, he knows Hogwarts, he knows Dumbledore and believe me, if he had been plotting all this time, that night when he marched up to Voldemort after 15 years of absence, he could have caused the fall of Hogwarts. Why? Because by then he would have made sure that he could come back with the Death Eaters as soon as possible to prove his faithfulness to Voldemort.  Snape is a genius in Potions and he likes the Dark Arts as well, so I deduce he's also a genius in the DA. Therefore, in 15 years he would have been able to find at least one loophole in Hogwarts if he had been set on conquering it. The Troy Horse, he would have planted one there.  Especially if he  had been working for himself:  he would have sent Voldemort to conquer Hogwarts before they were ready to take us on, and then he would let both sides kill each other until they were both weak. And then, if Snape had worked for his own agenda all that time, then Snape would be the conqueror!

But that is not what happened.  If I can deduce this, then Dumbledore could think for himself and he knew they would be doomed if Snape was not on their side.  Thus he needed an unshakable proof of Snape's loyalty. According to the information we have, this valid reason is either Veritaserum or an Unbreakable Vow.  I again vote for the latter because Veritaserum is good only for the present, you would have to retest someone each year to make sure they weren't hiding something. Could be done but it's such a hassle. And since kids like the Weasley twins knew at a young age of Unbreakable Vows, it seems like it's 'common knowledge' and should therefore be considered as the most probable solution. It could be anything else but I sure don't know what if it isn't the Unbreakable Vow.

What if Snape made an Unbreakable Vow to Dumbledore? What was or were the vows?

 Snape promised to do all that Dumbledore bade him to
Snape promised to protect Hogwarts and people against evil
Snape promised that he was repentant for his actions as a Death Eater

Whatever the reason, it will have to fit the plot which is why I came up with these three which, I believe, are in accordance with the books.  I'm not going to take the speculation farther now.

I want to focus on another logical thought I've deduced above but didn't discuss yet: If Dumbledore trusted Snape to not spill the beans to Voldemort, it means he knew that Snape was a wonderful Occlumens. Therefore, it follows that if Dumbledore needed a proof that Snape was on their side, he would NOT use Legilimens.  Voldemort thinks he's so hot and almighty that he won't believe someone can actually fool him through Occlumency. However, Dumbledore doesn't have that flaw.  So my point is that the way Dumbledore knows that Snape is not lying is NOT LEGILIMENS, he had to use something more drastic and safe.  Harry stressed the matter by asking the headmaster why he trusted Snape. He answered that it was between him and Snape, but also that he knew Snape felt guilty about revealing the prophesy to Voldemort.  If not for Legilimens, how would Dumbledore know for sure that Snape was in earnest?  Again I say the Unbreakable Vow and/or Veritaserum.  If Dumbledore used Legilimens on Snape and judged that he could trust Snape upon that when he answered Harry in HBP chapter 25, then it's the only proof we need to declare Albus an old senile wacko!  But I'm not Lucius, so I do not and will not believe that Dumbledore was crazy and that he made a mistake in trusting Snape. Because from that moment in the Infirmary (end of Book 4), Snape knew too much and Voldemort could have attacked a lot sooner even if he had not heard the prophesy because he would have been able to take on Hogwarts thanks to Snape's intelligence reports.  Too much was already at stake for Dumbledore, too many lives in the balance, he couldn't gamble like that because it could mean Voldemort's victory before the war even started.  

Dumbledore had known all year that they had to prepare in case Voldemort succeeded in coming back to life, ever since he first reappeared in Book 1 in fact.  Dumbledore is the incarnation of Good in the series, he would therefore plan way in advance when he saw that evil was lurking.  I'm sure he made plans with Snape way before Voldemort came back to life because Dumbledore likes to be well prepared, just as with the Philosopher's Stone.   And frankly, if I had been a Death Eater and I knew my old master was still alive and looking for ways to revive himself, I would run to Dumbledore's office and demand that we come up with plans in the eventuality that my maniacal master ever came back to life!  I wouldn't just sit there, waiting while sipping on a cup of tea, enjoying the biscuits and cakes while I knew my deranged ex-master was out there!  Snape has proven more intelligent than Gilderoy so I know they planned and schemed together.  What they came up with, we have no clue. But if was efficient enough that Dumbledore felt safe to let Snape go back to his ex-master and not perish or reveal essential information unwillingly. Like actors learn in theatres, like actors who perform after repeating and planning the play.  And Snape delivered the goods in chapter 2 of the Half-Blood Prince book!

In conclusion, Dumbledore trusts Snape completely: with his life and those of others, with Order information which he didn't even reveal to Harry until Year 6, with Hogwarts, with the wizarding world, with the Good side, with everything. Dumbledore has put the balance of the whole wizarding world of the British Isles on Snape's shoulders.  And you think that was a miscalculation?  A foolish decision?  To me, it is impossible that Dumbledore could hand the lives of an entire community into Snape's hands unless he had been dead sure he was a good guy.  In the Muggle world, you can't do that because there are no ways to really know if someone will betray you. But thanks to magic everything is possible!  

Now, was Snape this desperate when he turned coat?  Was he only trying to protect himself like all the others?  Then why did Dumbledore keep him as staff and a Head of House afterwards?  Why would a Slytherin put himself at risk? Very important questions indeed!  One even more important: why would he risk being sent to Azkaban by  admitting the truth to Dumbledore?

Remember that Slytherins will do anything to survive and avoid persecution.  That is how it is supposed to be anyway though I'm sure this rule is not as clear-cut as that.  But still, let's start from this: before the end of the first war against Voldemort, we know that those were still hard times for the Light (Order).  They were outnumbered. Even Lucius who is the epitome of self-protection found an excuse but only  right after the downfall of Voldemort and not before (he said he had been Imperioed so that stopped when Voldemort died only).  I'm sure Lucius couldn't come up with a more plausible reason because he wasn't expecting Voldemort to die. Nobody was or else Harry Potter wouldn't be such a great legend!  Nonetheless, Snape turned to the Light BEFORE the downfall of Voldemort.  Was he simply afraid that he wouldn't have a cover if Voldemort failed?  Unlike others, he knew of the prophesy – he literally knew a part of the prophesy – and he was intelligent enough to decipher it or was told by the Dark Lord himself what it meant and Snape thought: "Oh blast! I've got to find a way out of here if the prophesy is true!" Was he trying to have a backup plan in this eventuality? Snape was not in danger from Voldemort because he had just proven himself a faithful servant by telling him the prophesy, he was safe on that side. However, he wasn't safe if Voldemort died.  

Let's say this, and not guilt, was the reason why Snape went to Dumbledore.  What would the headmaster do?  What would Snape say? "Hello, I'm currently a Death-Eater but I feel very guilty  about it  (not really), I come to you even though you may feed me to the Dementors, which I hope you won't, because I really don't want that and I'll do some spying for you if you promise to protect me in the eventually that Voldemort dies. Well, he's not supposed to because you are outnumbered but just in case." Nope, not likely at all!  Even if Snape had come and  offered "a promise to be defended against my spying for you", it wouldn't have worked. First of all, why would Snape the Slytherin risk going to Dumbledore, a man who just had to snap his fingers to bind him and feed him to the Dementors?!  Why would he do that if he had nothing to fear from Voldemort?  No, Snape wasn't in danger there. Maybe he was tired of being a puppet, that could be it. But then, Dumbledore could not have accepted that.  Someone who was only trying to save his butt could not be trusted, not in a time of war, and guilt could not be manufactured. Snape knew that and he knew the headmaster was strong enough to force himself into his head through Legilimens.  If he did that, then Snape would have to show him scenes of guilt, but could he really invent them?  Could a cold-blooded killer who was only trying to save his butt feel guilt?  And since Dumbledore accepted Snape as a spy, it is logical to deduce that he knew Snape's Occlumency skills were higher than Voldemort's Legilimens skills. Which also means Dumbledore already knew he couldn't trust Snape only by looking into his mind. There had to be something more.  

Snape did that 'something more' (whatever it is if it's not an Unbreakable Vow or Veritaserum) and he was accepted as a spy and he begun spying.  That was Voldemort's original plan: send Snape to Hogwarts to spy.  Maybe Snape admitted he was a Death Eater when he met Dumbledore for an interview. Maybe the headmaster confronted him and he wasn't able to lie.  But why would a Slytherin who valued his life above all other things admit his wrongs so easily?! It's illogical and we know Snape is logical!  And what with his inability to lie, is that possible from a Slytherin? I don't think so.  I can't imagine Snape holding the Head of House position without the ability to tell a lie and know when he was lied to.  It's nonsense!  So that wasn't why Snape told Dumbledore.  No, Snape had a problem, he was in trouble and he needed help for some reason.   Maybe he did try to bargain because he feared Dumbledore would send him to prison right away, but then I'm sure that if that happened, Dumbledore won Snape's trust and he assured him that he wouldn't, and so Snape told of his problem.  Snape had to have a very good reason for knocking on Dumbledore's door and spilling all of his Death Eater secrets! A very good one which we'll only know in Book 7. I can't deduce further than what I have here already.  But Snape was ready to die for it and put his life on the line: he became a spy at great personal risk! What would be so important for him that he accepted to do that?  

The problem could be Snape's conscience, his love for Lily, his life-debt towards James Potter for saving him from the werewolf, his being threatened by another Death Eater (less likely) or simply because he trusted the prophesy and he knew the Dark Lord could vanish. But one fact remains whichever problem it was: Dumbledore kept Snape afterwards!  He kept him on his staff, he kept him close during Harry Potter's years, and now he even declares that he trusts him completely. How could Snape fool Albus to this point if he really is bad? Did Snape have no choice? Yes, he did because at the time Voldemort's power was great, Death Eaters were plenty and Snape had just given Voldemort the prophesy.  I see nothing except his conscience that would force him to become a spy. It certainly wasn't the fame!  Fame is nothing if you are found dead!  Protection if Voldemort died? I mentioned that and Dumbledore . Also, Sirius – who normally does not have a good word to say about Snape – said in Book 4 that he could not believer that Dumbledore would let Snape work at Hogwarts if he had ever worked for Voldemort. 

They say anti-social people have no empathy, it takes that to be able to not  feel bad about killing people.  I doubt we would find empathy in Lucius, Dumbledore knows that. But could Snape pull it off through Legilimency?  I don't think so. Dumbledore would not have felt it in a person who didn't feel guilty. But it seems he did in Snape.  And even if Snape had been able to pull it off, Dumbledore would require some sort of leverage, some sort of guarantee that he was really going to work for them in order for Dumbledore to intervene if Snape was caught. That is plausible. What doesn't make sense however is the reason why Dumbledore would keep such a mean, selfish, lying man on his staff!! It doesn't make sense at all.  Dumbledore would be way too happy to have gotten rid of such a bargain and wash his hands off him as soon as possible. But he didn't. More than that, he defended him and still does to this day.  My conclusion: Snape would not have worked at Hogwarts for so long if he had been bargaining his liberty in exchange of information.  No, Snape became a spy at his own personal risk because there was something more than 'saving his butt' that motivated him.  There was guilt and/or he was promised Dumbledore's protection if he spied.  I can't say if he volunteered or not (since he's a Slytherin, I doubt it but it could be).  But in order to be offered protection, Snape had to give a guarantee and not just any guarantee. Dumbledore was risking others' neck, and so the price had to be equivalent for Snape.  Which is why I believe even more strongly in the Unbreakable Vow theory because it lasts a lifetime and Dumbledore will put the lives of the population of the British Isles in the hands of Snape.  Wouldn't you say that the deal had to be as hard and long-lasting as diamond? I do!  It was a bargain that was balanced: Snape had to swear on his life that he would spy and would not reveal the Order's secrets or Dumbledore would have done without him. Dumbledore would have been prepared to do without Snape rather than take too much risk.

And so here is in a nutshell what I believe happened to Snape all these years (I won't include all details of course):  Snape went to Hogwarts and was sorted in Slytherin (this has been confirmed by Rowling herself).  He became a Death Eater some time during his last years at Hogwarts and before he got a job at Hogwarts. Snape, who was looking for a job at Hogwarts on the order of Voldemort (who surely knew of Snape's genius which is why he pushed someone so young to become a teacher). Snape  heard the prophesy, he was caught but still was able to tell Voldemort. Dumbledore may have summoned him in the meantime or Snape went back on his own .  He  asked  Dumbledore for help because he believed had made a mistake!  Either or both Snape's conscience or will to live made him go to Dumbledore.  The headmaster took Snape in, both as Head of House and Potions teacher, and he tried to protect the Potters even more.  But in order to accept Snape at Hogwarts, in a war where Aurors were outnumbered, Dumbledore had to tread carefully.  Could he risk it all and simply trust Snape's words? Nah!  It would be signing a death warrant!  Dumbledore used Veritaserum and/or an Unbreakable Vow (the latter is my favourite) and Snape swore to protect and work for the Light while being a spy at great personal risk. He accepted that and became the Potions Master and Head of Slytherin at Hogwarts.  He spied until Voldemort met Harry and 'died'.  Then he was formally defended by Dumbledore and he proved so trustworthy in fact that he  kept his job and is to be found near the headmaster and the deputy headmistress ever since.  He hated Harry Potter the minute he saw him, but he kept saving him no matter what. Then at the end of Book 4, he left to meet the Dark Lord who surely had a lot to ask him. He gave 'satisfactory answers' that dealt with all the loopholes, something Dumbledore and Snape must have spent hours devising, and Snape was back in the ranks.  He spied for the Order in Book 5 and taught Potter a bit. Snape alerted the Order when Potter was at the Ministry and this saved his neck.  

Over the summer, Dumbledore almost died because of the cursed ring Horcrux but Snape saved him.  Again why someone who had his own agenda or was a villain would do that, I cannot imagine.  If Voldemort knew of it, I'm sure Snape would end up on the receiving end of an Avarda Kedavra!  So, Book 6 begins and Snape explains the 'official' version of his story. But he was caught and couldn't weasel his way out of the Unbreakable Vow Narcissa proposed because it would further undermine his reputation amongst the other Death Eaters. (Go back to the analysis for more details on this part of Spinner's End) Anyhow, he thought the Dark Lord would ask him to finish the job (kill Dumbledore) if Draco failed so Snape knew that his days were numbered.  So he made the UV with Narcissa still because he knew that meant he would die for not helping Draco kill the headmaster.  Snape mentioned that to Dumbledore, they planned some more and Dumbledore said his life was less important than Snape's role in the Order. Snape tried to win Draco's trust because he wanted to 'control' the event and when it would take place, but Draco was not cooperative so Snape only learnt it when it happened.  Later in the year, Snape protested when Hagrid saw them together because it was either Snape or Dumbledore who had to die.  If Dumbledore had ordered Snape to kill him, he would not break the possible UV he made to Albus more than 15 years ago (if they made a pact, then it had to be worded carefully but it would allow Dumbledore to order Snape to kill him).  

Either way, Snape would die for not finishing Draco's job or for not obeying Dumbledore. Snape would rather drop dead than do that.  I understand perfectly!  And if Snape has had Dumbledore for a mentor or friend for so long, it would add to Snape's unwillingness to carry out the plan.  Not only that but it would make Snape's life a nightmare for a while. But I guess that was nothing compared to having to kill your friend. In the meantime, Dumbledore was telling Harry all that he needed to know, and he finished before setting off to the cavern.  Then he knew he could 'die' in peace. Dumbledore knew he may not come back from the cavern, I'm pretty sure of it.  In the meantime, it's a battle at Hogwarts, Hermione thanks to the lucky potion finds Snape to alert him, then he stuns Flitwick and imprisons them in the dungeons.  The Felix Felicis being a lucky potion, it is lucky that Hermione was not killed by Snape (according to Snape-the-bad theory) but it is a huge mistake if Snape is a villain because in all logicality Hermione should have known that she had to stun Snape and keep him there. She would have had so much luck that he would have tripped and lost consciousness right there. But that did not happen because luck wanted Snape to be able to go up where the battle was on.  Snape found Draco and no one aimed at him while he did this, then he sealed the tower so no one would tamper with Dumbledore's and Snape's plan.  Dumbledore arrived half-dead with Harry, he tried to convince Draco, another Death Eater was about to kill him when Snape came in.  Dumbledore pleaded Severus, there was a moment of silence which I believe was Legilimens, and Snape killed Dumbledore.  It was that or Snape would drop dead on the floor.  Albus was in no position to defend himself or protect Harry, and surely Snape knew that Harry was somewhere near.  Snape made his choice and he did it. Maybe he did it because he was obligated by Dumbledore's Unbreakable Vow as well, it could be. But we know that if he hadn't done it, Snape would have dropped dead for not helping Draco.

Snape takes Draco and they fly away. Draco is stunned, poor boy. Harry follows Snape and he discovers he's the Half-Blood Prince! (Yoohoo!) Snape teaches Harry yet another lesson about non-verbal spells (which if we follow the laws of drama will be just how Harry will be able to get Snape and kill him before he knows he's on the Good side, so let's hope Rowling hates the laws of drama ; ) Snape is insulted by Harry who called him a coward because what he just did was the most difficult thing in the world!  Snape goes into hiding, and we wait for Rowling to send her finished copy to the editor!!

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There is another point which is essential to make clearer as well: the DADA job. Why did Snape only get it in his last year if evidence that Dumbledore trusted him completely was given?! Here are my hypothesis explaining why Snape got it only in Year 6. It furthers confirms my theories that Snape is not a bad guy.

the position truly is jinxed and that Dumbledore doesn't want to risk Snape teaching it in case he would also be unable to teach it anymore or die because of the jinx. I mean, how come that ever since Voldemort was refused this position that no one was ever able to keep the job for more than a year?!  That means more than 16 years in a row!  A wonderful and elaborated coincidence by Rowling or a truly jinxed position for anybody but Voldemort?
the position is jinxed but Snape is immune because Voldemort ordered him to get a job  there but he didn't mention "Oh and by the way, don't take the DADA one, I've jinxed it so you would only be able to stay a year, and that wouldn't help our cause, would it!" No, Voldemort would have made sure he could put someone he wanted there if he truly jinxed the job. I'm thinking of anyone with the Dark Mark or someone with evil intentions.  Evil works with Snape because we still don't know if he's working for himself, Voldemort or Dumbledore.  In any event, this option made sure that Snape was an exception to the jinx (through the Dark Mark most probably). However, if Snape knew he could take the DADA job while others couldn't, it's also probable that Dumbledore knew that and that it would look suspicious that only Snape was able to keep the job for so long, hence people could start talking and link Snape with Voldemort again. And we know Dumbledore sure didn't want that out of any of his staff. So Dumbledore would be forced to not give Snape the job or else it would be too obvious Snape had something special and dark.  That's a long shot but I had to mention it.
Dumbledore fears a relapse in Snape and considers that even more important than giving the kids a really efficient and long-term DADA teacher
Dumbledore thinks Snape is better at Potions and he can't do without him because there are fewer Potions Master than DADA teachers
Dumbledore was required by the Ministry that Snape never become a DADA teacher but he had to in Year 6 because having Slughorn on the staff was even more important than Snape having the DADA job and whatever consequence that might give rise to (a relapse, Ministry disapproval, etc)
A mix of two or more of those hypothesis

I personally think that the most plausible reason is a combination of hypothesis. I think that Dumbledore had a lot to think about and chose not to give Snape the DADA job for a couple of reasons which, when balanced, seemed better than giving him the job up until Year 6. Here are the main reasons I believe in. Oh and don't forget this: "Dumbledore trusts Snape completely - which in effect means with his life!" and Snape did get the job in Book 6:

Dumbledore can't do anything about the position being jinxed (except maybe if he gives it to Snape or someone wearing the Dark Mark), but he doesn't know if he should allow Snape a go at it because of the jinx. He doesn't want to lose Snape as a staff member since all the others didn't seem to stay at Hogwarts afterall. It may also be difficult for Snape to find a job elsewhere for whatever reason (though I'm more skeptical about that).
Snape is also fragile in terms of the Dark Arts, so it would reinforce the fact that Dumbledore fears for Snape's mental health.
Snape is not your dream-teacher (okay, okay, don't sue me ladies, not your dream-teacher for most people out there!) so Dumbledore wouldn't want him to have the job before he changed his attitude. Maybe that's why or he simply doesn't trust Snape to be a good DADA teacher.
Year 6: something happens in this year and it's suddenly more important to have Slughorn on staff than whatever reason Snape could not get the job before.  It also doesn't matter that Snape is the new DADA teacher.

Allow me a little experiment here. I'm going to think like an employer and someone who cares for Snape (I think Dumbledore does) because I want to clarify the inconsistency of giving Snape the DADA job in Year 6:  

If I were the Headmaster and I had Snape on my staff, I would want to make sure he didn't relapse, especially in an institution meant to protect and educate the youth of the wizarding world.  That's fine and I would really insist on it in the first years following his turncoat behaviour on the Death Eaters.  I would keep track of his progress, too. But then, after the dust had settled, there would be some years when the risk of Snape falling back into his old habits would be lessened because not a lot of people would talk about it anymore. I mean some years before Harry Potter came to Hogwarts, some years of peace and quiet in which the Dark Arts weren't so threatening anymore as they lacked a grand leader. A time when even Snape's ex-Death Eater friends would not be too bent on the Dark Arts because they would easily be caught. Snape did mention he had to play safe because they were rounding up the Death Eaters. However, this sounds a good excuse only in the first months or years after Voldemort fell. Afterwards, many ex-Death Eaters had important roles in society and it looks as though parents who might have objected to Snape being at Hogwarts, could not do so anymore. Snape was accepted in Hogwarts, period.   So...

Why didn't Snape act then if he truly had his own agenda? Why not act when everyone was still weak from those years of battle? If Snape had any thoughts of becoming the next Dark Lord or to place himself above all the others thanks to his abilities, why didn't he do it then? He simply had to go in hiding and work his magic from there. He didn't have followers so people wouldn't have been afraid of him, you say?  Has Snape ever wanted followers? Has he even an underling beneath his foot like all normal villains? No.  He has no one following him like a servant would. Nada, niet, zenzen, rien du tout!  Okay, maybe Snape is hiding those 'servants' but I doubt it because there wasn't even a small hint about it in the six books, and Rowling always leaves clues. And this, my friends, is why I do not believe that Snape has his own agenda.  Yes, he could be only waiting for the rope Dumbledore and Voldemort put around his neck to break like a bad dog on a leash, but what if said leash was unbreakable?  What if he had tied himself up for Dumbledore as a proof of his loyalty to him?  Or to Voldemort? I doubt the latter because all the Death Eaters seemed to walk freely after his demise.  But a promise to Dumbledore... I think this is the reason why he trusts Snape with his life. Because even if Snape wanted to break his rope and run away, he would not be able to.

If Snape has made no other Unbreakable Vow however, yes then it's easy to hypothesize that Snape is working on his own while pretending to work as a double agent. But it still wouldn't explain why he hadn't made a move yet!  Why let Harry Potter live if he logically represented the worst obstacle to any Dark Lord-to-be? I don't buy Snape's reason (see in the next paragraph of the text). I don't believe Snape lost all of those chances at greatness just because 'it wasn't the right time' or because 'Dumbledore was looking over my shoulder'. That's not like Snape, I don't buy it.  Snape wasn't physically hurt  when he turned over to Dumbledore, was he? If so, then why not flee as soon as possible and go conquer some other part of the world saying: "Oh, I'm tired of teaching dunderheads, I want to explore the world. Good bye, Albus."  Why didn't he do that?  Surely he would have done something else if he could... but he didn't. Snape stayed at Hogwarts, very faithfully so I might add. Even if the DADA job was not within his reach all of those years.

In the meantime, dark wizards could have been looking for another powerful figure. It could have been Snape because he is very astute and clever. He's a pro at both Potions and Dark Arts. But no, Snape didn't seem to be interested in that or else he would have taken his chance by now.  So in those peaceful years it would logically follow that Snape would not be so fragile to the Dark Arts because they were not so present. But as a friend and/or boss, I would still keep him where he is and not on the DADA job because I feared the job might be jinxed and/or it would make Snape go back to the dark. However, I trust Snape with my life so why would I fear that?  When you trust someone that much, you don't fear their going overboard. You know they will protect you no matter what and at the risk of their own life.  So why be so sorry about the DADA job?  I don't think it's because Snape might fall back to the dark side.  There is another very important reason that became of no consequence by the time of Book 6 when Snape finally got the job.

But first let's talk about when Harry Potter came to school. During his first year, as the headmaster, I notice that Voldemort has risen again. I know he's alive and will try to contact his old followers. My first reaction: my Potions Master is at risk!  Especially if someone tries to prove he is working only for me now. Snape would be at risk because he worked against Voldemort in Book 1, and he didn't kill Harry Potter afterwards even though he knew Voldemort was not really dead and could retaliate later on. From now on, Snape and I would be on our guard and we would discuss what we would do if Voldemort was able to be revived which takes us to the end of Book 4 when I ask Snape if he's ready to do what he must.  But if I have been very concerned for Snape not being able to cope with the Dark Arts without falling back to his old habits, why would I suddenly feel okay to send him to the Death Eaters and Voldemort in Book 4? Isn't that very illogical!?! I've spent years protecting Snape from the Dark Arts and there I am, sending him off to his doom (if Voldemort doesn't accept his apologies) and to the Dark Arts.  It doesn't add up!  And it goes on until the end of Book 5, too. The Death Eaters, to whom my Potions Master once adhered, officially came back in power by the end of Book 5. So to encourage my dear Potions Master who has to spy for me and for whom I have feared a relapse, I give him the DADA job!  

This is all wrong because if I use that same logic of keeping Snape as far as possible from the Dark Arts, I wouldn't give him the job, especially now that his 'old pals' are on the move and that I am using him as a spy amongst their midst. As a spy he would witness atrocities and  use the Dark Arts surely, so if I had been keeping him from harm's way all of those peaceful years, wouldn't I do the same now? Would I not worry that he would go back to his old ways behind my back? This doesn't make sense! You don't send your ex-alcoholic back to the pub if you were so unsure of him when he was in your home.  And that is why I say Dumbledore has all the reasons to trust Snape, but not because he's fragile when it comes to the DA.  

And why did Snape return? To spy for the light? Or for the simple fact that Voldemort would have had his head if he hadn't come back crawling for mercy. But then, Dumbledore always foretold the revival of Voldemort and Snape would have known that, too. So why would he do that if they both believed Voldemort would one day grow back to power in the first place? Why would he have isolated Snape all those years only to feed him head first to the Death Eaters in Book 4!?  That's illogical and it lacks finesse and strategy. And we all know how well Dumbledore moves his pieces on the chess board.  And his next move was to give Snape the job.  However, if I put myself in the headmaster's shoes, Year 5 was already difficult but I didn't resort to give him the DADA job then. There's something that happened over that summer or Year 5 that meant I had to give him the job. And that would be even more important than making sure Snape wasn't going back to his old ways if this was the only problem before.  What is it? Hiring Slughorn. Yes, that works fine.  Making sure Snape pleases his master by telling him that he finally, at long last, got the DADA job? Yes, that's also true and then if the position was truly jinxed, Voldemort would make sure that Snape would not be expelled after only a year if it could be helped. But then it was at the end of Book 4 that Snape needed a very good news for Voldemort to take him back in his good favours instead of executing him, not in Book 6! The DADA job would have been a bonus to help Snape at the end of Book 4.. But no, it was not then that he gave it to Snape.

I think that the remaining reasons why Dumbledore had to give Snape the job in Year 6, no matter what he feared for Snape in the first place, were Slughorn and the Unbreakable Vow.  Snape's days were numbered when he made the UB.  He had two choices: kill Dumbledore or suicide/give up his life by not fulfilling his vow because he didn't want to kill Albus. And there lies the duality of Snape, the one that the whole Harry Potter world is now wondering about! It's nice to know we have been focusing on a very decisive character from the beginning ; )

To us, the UVow is primordial because it means life or death for Snape in Book 6. That's what I shall try to analyze through the rest of this chapter.  Now what we must remember is that it's very hard to make sense of everything with so many possible outcomes. However, I think it is very important to remember  these points about DADA job business:

-Dumbledore trusts Snape completely, hence with his life, hence he sure has a very very very good reason to do so. If not, then Dumbledore wasn't as hot and wise as we thought and I rather doubt that.

-The quality of the DADA class seems less important than whatever it would do to have Snape on the job. Because I truly believe he would be very good at it because he's been there, he knows so much, and he has survived that long because he knows exactly how to defend himself.  But since Snape didn't get the job and was overlooked in favour of numerous other 'teachers' who got the job for less than one year, I think it's essential that we remember that the quality of THE most important class at Hogwarts is of no concern to Dumbledore compared to keeping Snape where he is.  It is so important to keep Snape either sane or at Hogwarts that Dumbledore refuses to give him the job.

The only other realistic possibility I can come up with to explain why Snape only got the job in his last year is for many of the same elements I used in my Story in a Nutshell above: after Snape made the Unbreakable Vow with Narcissa, he discussed with Dumbledore about it. Then they both decided that Snape had to carry it off because if it wasn't Snape who would kill Dumbledore, it would be someone else anyway so might as well put Snape in the spotlight! Snape couldn't be lost because he was the best asset of the Order, even more important than Dumbledore being alive, especially now that his body was tainted by the ring-Horcrux that he destroyed before.  Dumbledore needed Slughorn, Snape needed a bit of a boost to his morale, so he gave him the DADA job both out of necessity and to 'compensate' Snape for what he would have to do in the end. Or because, if the job was jinxed, Snape wouldn't last a year anyway, so might as well enjoy his last year at Hogwarts and prepare Harry in the meantime!  

And if Dumbledore had made sure to inform Harry of all he had to know before his demise, this means that the rest of the information held by the headmaster was not secret or he passed it on someone else.  A someone else that is believed to be Albus's brother by many fans or even Regulus Black by others or my personal favourite: Albus himself as a portrait. Whomever this is going to be, Dumbledore seemed confident enough to be killed without having to worry in the after-death about his successors.  Anyone who has worked in security or lived through a war will know that all good stratagem has a back-up plan!  If Dumbledore didn't have one, then again I say he should put his name down to compete in next year's Nutty Balmy Crackbrainy Dotty Batty Tournament held at the Daft, Demented & Wacky Festival down in Delusional country.